1. Field of the Invention
The invention relates to an amplifier circuit for use, e.g., in audio appliances and, more particularly, to an improved amplifier circuit which can reduce the voltage distortion rate of an output power.
2. Description of the Prior Art
Recently, many developments have been made to reduce the output voltage distortion rate of an amplifier circuit which controls the output power by a negative feedback. But still, none of the conventional amplifier circuit has accomplished to lower the output voltage distortion rate substantially to zero. This is especially difficult for an amplifier circuit which supplies electric power to a load, because the linearity of an output power characteristics will not be maintained when a great amount of electric current flows through the load. Thus, the output voltage distortion rate becomes poor.
FIG. 3 shows an example of a conventional amplifier circuit in which 1 is an input terminal; 2 is an output terminal; 10 is a preamplifier; 11 is a predriver transistor; 12 is an SEPP (single-ended push-pull) output stage; 9 is a negative feedback circuit for the negative feedback of the output voltage; and 13 is a phase compensation circuit.
The gain G of the amplifier circuit in this construction is shown as follows: ##EQU1## where A.sub.1 is the gain of the preamplifier, and A.sub.2 is the gain of a circuit covering from predriver transistor 11 to SEPP output stage 1. A.sub.2 can be expressed by the following equation: ##EQU2## in which r.sub.e is the emitter resistance of predriver transistor; Z.sub.p is the impedance of phase compensation circuit 13; h.sub.fe is the current amplification of SEPP output stage 12; and Z.sub.L is the impedance of the load to be connected to output terminal 2.
The open-loop transfer characteristic of this construction is as follows: ##EQU3## in which T.sub.1 and T.sub.4 are the time constants of negative feedback circuit 9; T.sub.2 is the time constant of preamplifier 10; T.sub.3 is the time constant determined by phase compensation circuit 13; and T.sub.5 and T.sub.6 are the time constants determined by SEPP output stage 12.
To maintain the stability of the feedback circuit, the parameters are determined as follows:
T.sub.1 &gt;T.sub.0, T.sub.2 &gt;T.sub.0, T.sub.3 &gt;T.sub.0, PA1 T.sub.4 &gt;T.sub.0, T.sub.5 &gt;T.sub.0, T.sub.6 &gt;T.sub.0
provided that T.sub.0 =1/.omega..sub.0, and .omega..sub.0 is .omega. evaluated at A.sub.1 .multidot.A.sub.2 .multidot..beta.=1.
According to the prior art amplifier circuit as described above, the electric current to be supplied to the load flows through the SEPP output stage. Also, the current amplification h.sub.fe of the SEPP output stage generally decreases when the electric current increases. Accordingly, the current amplification h.sub.fe varies with respect to the change of the output current. Thus, the gain A.sub.2 becomes nonlinear, resulting in the electric power distortion.
Although it is preferable to make .omega..sub.0 greater to improve the output voltage distortion by the negative feedback, .omega..sub.0 can not be made greater than a certain level due to the increase of T.sub.5 and T.sub.6 in a region of high output current. Thus, the voltage distortion cannot be effectively improved by the negative feedback in the high frequency regions.